The present invention relates in general to an imaging system for providing co-registered images of objects to a plurality of separate cameras, detectors or other optical imaging devices.
Numerous imaging applications require generation of multiple, co-registered images to facilitate multi spectral analysis thereof. As an example, stress in growing plants can be monitored, in part, by observing variations of the plant's reflectance of a specific radiation wavelength, 700 nm, which is readily absorbed by chlorophyll. If the plant is under stress, its chlorophyll production decreases, and its absorption of 700 nm wavelength radiation also decreases. Relative levels of chlorophyll in a plant can therefore be determined by measuring the plant's reflectance to 700 nm wavelength radiation. However, other environmental factors, such as ambient light level, also affect the plant's reflectance characteristics. Thus, the plant's reflectance to a reference wavelength, which is not affected by variations in chlorophyll production, must also be employed as a comparison standard. This necessitates the generation of two co-registered images of the plant, one of which is limited to light having a wavelength of 700 nm, and the other of which is limited to the reference radiation wavelength. By analyzing the ratio of the two reflectances, a determination of the chlorophyll production in the plant can be made.
In other situations, it is desirable to combine different types of images to provide the observer with more visual information. For example, optical fire detectors employed by NASA at the Kennedy Space Center employ infrared responsive image detectors to detect hydrogen flames because such flames are invisible to the naked eye. The infrared images cannot, however, provide a visible indication of the actual location of a detected fire. Thus, a visible spectrum image of the monitored area must be overlaid with the infrared image to provide this information.
Previous techniques for generating multi spectral co-registered images typically rely on the use of single element detectors with either scan mirrors or sensor motion relative to the target to produce a two-dimensional multi spectral image. The image of a single picture or image element, i.e., pixel, is focused on a field stop, re-expanded by a collimator, wavelength dispersed by means of beam splitters, dichroic mirrors, prisms, gratings, etc., and refocused onto single element detectors. The signal from each of these detectors is simultaneously digitized and stored as digital data. Each image pixel then consists of a data value for each wavelength. Since the image can only be formed by scanning sequentially, none of the pixels are coincident in time, or as in the case of a linear array of detectors, no two lines of the image are coincident in time. This requires post processing by a computer and software with significant data storage capability, or some special processor to process the data and form an image for presentation to an end user. Furthermore, even if the optical elements in one of these systems were specifically configured so that the resulting images could be easily combined, any reconfiguration of the lenses, beam splitters and filters would require a substantial redesign of the system components.
In view of the foregoing, a need therefore exists for an optical imaging system which generates co-registered multi spectral images without the need for extensive data processing, and at the same time permits reconfiguration of lenses, beam splitters and filters without redesigning the system components.